Cell fusion is required for many aspects of development and differentiation. The formation of multinucleated cells during myogenesis in the Drosophila embryo is an ideal model to study the genes required for cell fusion events. One of the genes required for myoblast fusion, Myoblast city (MBC), is part of a conserved signaling pathway required for reorganization of the cytoskeleton. This proposal aims to identify new players required for Drosophila myoblast fusion in the MBC signaling pathway. Specific Aims: 1) To determine whether ced-12 is an essential component of the mbc-related pathway for Drosophila myoblast fusion. 2) To test the hypothesis that phosphatidylserine is required, with or without MBC, for the fusion process. 3) To identify and characterize additional binding partners of MBC and determine whether they are required for myoblast fusion. These studies will examine the involvement of candidate proteins as well as identify new proteins required for cell fusion in myogenesis. Characterization of these proteins will provide a better understanding of muscle fusion and provide a framework to better understand molecules required for cell fusion in general.